Dry electrical transformers are advantageous over oil filled electrical transformers in several respects. Dry transformers do not require periodic maintenance and oil replacement as in the case of oil filled transformers. Oil is environmentally polluting and capable of causing health hazards besides being susceptible to fire accidents. Dry transformers are preferred for outdoor applications because of their properties like resistance to ultra-violet rays and moisture, flame proof nature or excellent insulation characteristics. Dry transformers generally operate at higher temperatures of the order of 120 to 180° C. and are preferred in hazardous areas such as mines, densely populated residential areas or hospitals. Dry transformers are also without the protective metallic tank required by the oil filled transformers.
We have described in our PCT Publication No WO 2006/016377 (based on Indian Patent Application No 307/MUM/2003 filed on 26 Mar. 2003) a compact dry transformer comprising a resin impregnated or encapsulated coil assembly. In order to increase heat dissipation and cooling efficiency of the transformer, heat sinks are provided on the core, between the core and low voltage winding (primary winding), between the low voltage winding and high voltage winding (secondary winding) and over the coil assembly. Further experiments and findings have shown that the resin impregnation or encapsulation of the windings offers resistance to the flow of heat from the windings to the heat sinks adjacent to and within the windings thereby reducing the cooling efficiency of the transformer. Heat retention in the windings over a period of time may damage the windings and reduce the life of the transformer. Also provision of heat sink within the coil assembly between the low voltage winding and high voltage winding increases the size of the high voltage winding correspondingly increasing the material cost and weight thereof. In order to ensure adequate resin impregnation between the layers of conductors of the windings, sufficient clearances are to be provided between the layers of conductors of the windings. As a result also, the size and weight of the transformer are increased.